As a society, we often take for granted the mobility (power and range) afforded by the energy storage density of common transportation fuels such as gasoline, aviation kerosene, and diesel fuel. The legacy investment in the refueling infrastructure alone makes it apparent that fuel cell technology capable of utilizing these existing fuels may have a distinct advantage over those restricted to high purity hydrogen or other less widely available fuels. The ability to utilize reformate produced from these existing transportation fuels, as well as from emerging non-petroleum based fuels such as bio-diesel, and synthetic (Fischer-Tropsch) liquids, without the need for extensive cleanup is an advantage of solid oxide fuel cells (SOFCs).
The higher efficiency of fuel cells compared to conventional engines is one of the main characteristics motivating the development and eventual commercialization of fuel cells. In stationary applications, utilizing natural gas fuel, this efficiency advantage is well established. However, where liquid fuels are used, a fuel processor used to reform liquid fuel exacts a heavy efficiency penalty on a fuel cell system. Historically, the sulfur and aromatic content of transportation fuels has made them impossible to reform using the catalytic steam reforming process used with natural gas systems, due to problems with “poisoning” the catalyst and carbon buildup. As an alternative, partial oxidation processes (e.g., PDX, CPDX, ATR, etc.) have been employed, but these also suffer drawbacks.
Although reformed fuel or “reformate” produced by conventional partial oxidation of certain fuels typically represents about 80% of the energy content of the fuel as measured by heating value, the use of conventional partial oxidation processes with fuel cells results in a loss in the range of 30 to 40 percent of the electric power generation potential of the fuel. This is primarily due to the fact that a fuel cell is not a heat engine. Rather, a fuel cell may be considered a Faradaic engine, and the Faradaic (current producing) potential of a fuel cell is reduced by 4 Coulombs for each mole of O2 introduced during the conventional partial oxidation process.
For example, referring to FIG. 1, in general, a prior art system 100 for producing electricity using a feedstock fuel 106 as an input may include a reformer 102, or fuel processor 102, and a fuel cell 104. The reformer 102 may receive and process a hydrocarbon feedstock fuel 106 to produce synthesis gas 112 containing a mixture of carbon monoxide and hydrogen gas. This synthesis gas 112 in addition to oxygen 114 may be used by the fuel cell 104 to produce electricity 116. In certain embodiments, the fuel cell 104 may generate CO2+H2O 118 and heat 120 as a byproduct.
Where natural gas or methane is used as the feedstock fuel 106, a reformer 102 may utilize a process such as catalytic steam reforming (CSR) to produce synthesis gas 112. This process generally involves reacting the methane with steam in the presence of a metal-based catalyst to produce the desired synthesis gas 112. CSR and similar processes, however, are unable to reform liquid transportation fuels such as conventional diesel, heavy fuel oil, or jet fuel (e.g., JP-8, Jet-A, etc.). This is because the sulfur and aromatic content of transportation fuels makes them difficult or impossible to reform using CSR, at least in part because of problems with “poisoning” the catalyst and carbon buildup. Instead, partial oxidation processes (e.g., PDX, CPDX, ATR, etc.) are normally employed to reform transportation fuels.
In general, a conventional partial oxidation process may include partially combusting a sub-stoichiometric mixture of feedstock fuel 106 (which may include chains of —CH2— groups, or more generally CHn groups) and an oxidant 108. The combustion reaction is exothermic and provides heat 110 utilized in reforming the remaining fuel 106 to generate synthesis gas 112, the reformation reaction of which is endothermic. The heat of reformation is on the order of 30 percent of the heat generated by completely combusting the fuel 106, which can be obtained by partially combusting the fuel. Where fuels 106 are high in sulfur content, partial oxidation reactors may employ non-catalytic partial oxidation of the feed stream 106 with oxygen 108 in the presence of steam at temperatures exceeding 1200° C.
The stoichiometric reformation reaction occurring at the reformer 102 and using oxygen 108 as the oxidant may be represented generally as follows:—CHn—+(1/2)O2→CO+(n/2)H2 At the fuel cell 104, the synthesis gas 112 and oxygen 114 is converted to electricity 116, carbon dioxide 118, and steam 118 in accordance with the following equation:CO+H2+O2→CO2+H2O+4e−As can be observed from the above equations, each CH2 group generates about 4e− (4 electrons) of electricity using a conventional partial oxidation reformer.
Conventional partial oxidation techniques exact a heavy efficiency penalty on the fuel cell 104. The Faradaic (current producing) potential of a fuel cell 104 is reduced by 4 coulombs for each mole of oxygen 108 introduced in the partial oxidation process. Thus, the oxidant in convention systems reduces the ability of the fuel cell to produce electricity.
In view of the foregoing, what are needed are an improved system and method for generating reformate from various fuels that improve the Faradaic efficiency of fuel cells, such as solid oxide fuel cells (SOFCs), molten-carbonate fuel cells (MCFCs), or phosphoric acid fuel cells (PAFCs). Such a system and method would be capable of reforming fuels with high sulfur content (e.g., 10,000 ppm) without requiring sulfur pre-removal, while avoiding problems such as “poisoning” the catalyst or carbon buildup. Further needed is an improvement to the overall efficiency of fuel reformation and electricity production.